Variable Frequency Drives (VFDs) frequently use Insulated Gate Bipolar Transistors (IGBTs) which may be replaced by wide band gap devices (SiC and GaN) as the ultimate choice for power semi-conductor switches in Voltage Source Inverters. These devices have extremely fast rise time and fall time in the order of nanoseconds compared to microseconds in the presently used IGBT devices. The fast rise time and fall time result in extremely high dv/dt of the output voltage being fed to an AC motor. The high dv/dt causes significant leakage current that can cause premature failure in the cables carrying power to the AC motor. When the distance between the motor and the inverter is long and there exists a mismatch between the cable and motor surge impedance, there is voltage amplification at the motor terminals due to the high dv/dt. This is attributed to a phenomenon known as “Voltage Reflection”. The high dv/dt of PWM outputs has been found to create excessive voltage stress in the insulation system of AC motors. An effective technique of mitigating over-voltage at motor terminals and reduce leakage current in motor cables and through the frame of the AC motor is by employing an output dv/dt filter.
In many oil field applications, the distance between the motor and the Variable Frequency Drive (VFD) approaches 300 m. Traditional dv/dt filters have been found to be inadequate in reducing the over voltage at the motor terminals. The damping resistor often experiences high voltage and gets damaged.
In practice, the over-voltage at the motor terminals, which depends on the distance between the motor and the inverter as well as on the impedance mismatch between the cable and the motor surge impedance, can reach as high as double the DC bus voltage of the inverter. In certain cases, due to very short transition time between modulating pulses, the peak transient appearing across the motor windings can be as high as three times the DC bus voltage. The high rate of rise of voltage pulses in the range of a few tens of nanoseconds give rise to voltage reflection phenomenon that can cause high voltage transients to appear across motor terminals that eventually leads to damage of insulation and consequently failure.
The high dv/dt at the output of PWM inverters has been shown to cause two distinct types of problems in a motor insulation system. The first type of problem can manifest itself as an insulation breakdown of the first few turns of the winding. This is purely due to the high voltage gradient applied across the first few turns of the winding. The second type of problem encountered is pinholes in the insulation due to corona discharge caused by high dv/dt of the applied voltage.
The phenomenon of reflection of electromagnetic waves on an electrical conductor is similar to a wave in water. At a given point, the magnitude of the electromagnetic wave varies with time and its phase is retarded. If a load on a transmission line is physically at infinite distance from the source, there exists no reflection. One can electrically create an infinite transmission line (a line having no reflection) if the surge impedance at the terminating end matches the cable surge impedance. However, in most cases the motor and cable surge impedance are mismatched which causes voltage reflections. Voltage reflection further could potentially cause voltage amplification at the motor terminals due to the additive nature of incident and reflected waves.
Smaller horsepower motors have larger inductance and less amount of slot insulation, which results in a larger surge-impedance compared to high horsepower motors. Hence, the mismatch between cable and motor surge impedance is the highest in small motors.
The over-voltage at the motor terminals due to long lead lengths can deteriorate the insulation system of the motor resulting in premature motor failures. Three different techniques are discussed in this section, which help alleviate the over-voltage problem encountered by motors at great distances from drives. They are: (i) Use of load reactors; (ii) Use of RC snubber at the motor terminals; and (iii) Use of an RLC filter where LC forms the dv/dt slope changer and R is across L to damp out the oscillations caused by the LC and cable combination.
By using 3-phase load reactor in between the motor and the PWM inverter, one can change the characteristic impedance of the motor or that of the source depending on where the inductor is physically placed. Adding a load reactor at the motor end will result in altering the surge impedance of the motor. The inductance component of the surge impedance of the motor is artificially made high which causes the overall surge impedance of the motor to be higher than normal. The mismatch between the surge impedance of the motor and the cable is aggravated thereby resulting in a higher coefficient of reflection (ρR) and a higher voltage at the terminating point. Since the terminating point now has the 3-phase inductor, the over-voltage is experienced by the windings of the inductor instead of the motor. The negative influence of adding an inductor at the motor end is that the reflected voltage traveling along the conductor back to the sending end will now have higher amplitude and will need to be absorbed by the sending end. In many applications, it is impractical to add a mitigation technique at the motor end.
Adding a load reactor at the inverter end will result in altering the surge impedance of the cable. Typically, the surge impedance of the cable is lower than that of the motor. By increasing the surge impedance of the cable artificially, the coefficient of reflection, ρR is made lower which reduces the magnitude of the reflected wave. The increase in the effective impedance of the cable can be achieved by placing the inductor at the drive end. Typical value of impedance used is 0.03 p.u based on fundamental ratings.
Artificially increasing the surge impedance of the cable by adding an output inductor at the inverter end also causes the mismatch between the cable surge impedance and the source surge impedance to increase thereby increasing the amplitude of reflected wave at the inverter end. However, the main voltage stress is absorbed by the inductor rather than the VFD.
The RC filter is one of many methods that have been used in the industry. In its simplest form, it consists of resistors and capacitors configured as an RC snubber. The RC snubber is typically installed at the motor terminals and acts as an impedance matching network. Its performance is dependent on the inductance of the cable and is very important in order to predict the effectiveness of this type of filter. The snubber components are carefully selected to absorb the voltage spikes occurring at the motor end due to voltage reflection issues. For transient rising and falling edge of the PWM voltage waveform, the capacitor behaves like a short circuit and allows current to flow through it. The resistor in series with the capacitor dissipates the energy flowing into the RC network and thereby damps out the oscillations. The voltage spikes are thus snubbed by the RC snubber circuit at the motor end.
Though the idea is sound, the value of R and C needed in the snubber circuit depends on the amount of energy that is needed to be dissipated, which in turn depends on the distance between the motor and the drive and the characteristics of the cable. In addition, due to logistical and environmental reasons, it is difficult to access the motor terminals in many applications. Given these facts, the wide spread acceptance of the RC snubber method is limited.
The snubber network is intended to absorb the voltage transients caused by voltage reflection at the motor end. The major disadvantage of this method is that the network is effective only when employed at the motor end. This makes it impractical in many applications since installing it at the motor end may not be possible due to environmental and other logistical concerns. Also, it is necessary to select C properly to effectively absorb the transient. Since the value of the peak voltage at the motor end depends on cable length and cable characteristic, it makes its effectiveness unpredictable. Finally, the value of the damping resistor depends on the value of C used and so it is indirectly dependent on cable characteristics. The wattage of the resistor can also be large depending on the voltage that needs to be absorbed. The heat dissipation in the filter near to the motor terminal needs careful planning and consideration.
The traditional dv/dt filter consists of an RLC filter interposed between the inverter and the induction motor. This method reduces the dv/dt at the motor terminals that help reduce the voltage reflection issue at the motor terminals. It also helps reduce the shaft voltage and common mode current that are important in reducing bearing currents, and conducted EMI to some extent.
The dv/dt filter is used to modify the rise time and fall time of the PWM voltage waveform and unlike adding an inductor alone, the dv/dt filter does not alter either the cable surge impedance or the source surge impedance. Modifying the rise time and fall time using dv/dt filters has limitations since the cable inductance and capacitance influence the final value of the voltage transient at the motor terminals.
There are two types of dv/dt filter. In the first type, an inductor is used at the output of the drive and a capacitor-resistor network is connected either in wye configuration or in delta configuration at the output of the inductor. Motor cable is connected to this output. In this topology (resistor in series with the resonating capacitor), the effectiveness of the circuit is reduced since the series resistor adds impedance to the leakage current, which diminishes the capability of the circuit to shunt the leakage current. Further, the value of the series resistor should be large enough to provide sufficient damping but at the same time should be small to reduce the slope of the voltage across the RC combination. This contradictory requirement makes the choice of a suitable capacitor-resistor combination difficult. The lead length between the inverter and the motor also influences the performance of the circuit. The dv/dt filter with the damping resistor in series with the filter capacitor is shown in FIG. 1.
A popular alternate topology is shown in FIG. 2. In this topology, the damping resistor is placed in parallel across the filter inductor instead of the series RC configuration of FIG. 1.
The advantages of the dv/dt filter with the resistor across the filter inductor are that: the output dv/dt filter is employed at the inverter end and easy to install; this filter reduces the rise time and fall time of PWM Pulses to a mild level (typically 1 μs) both in the normal and common mode voltages; and problems related to the leakage current, shaft voltage and surge voltage at the motor terminal will be mitigated by this filter. Reduced RF noise will be expected to some extent.
The filter shown in FIG. 2 could also have issues when the distance between the motor and the drive is quite long. The capacitance in the cable can also influence the effectiveness of this topology. When the cable capacitance is large, the cable surge impedance is small. The cable surge impedance is now much smaller compared to drive surge impedance in the presence of the R-L filter section of the dv/dt filter shown in FIG. 2. This can cause the reflected voltage at the filter to be higher than usual causing higher power loss in the damping resistor, since the other end of the inductor-resistor combination is clamped to the DC bus.
The present invention is directed to satisfying the requirements discussed above, in a novel and simple manner.